An overcoat layer is formed for protection or insulation of an electrode, a resistor body, and the like formed on a soda lime glass substrate, an alumina substrate, or the like. A powder material paste has hitherto been used for formation of the overcoat layer. In general, the powder material paste is a mixture of glass powder and a vehicle, and as required, ceramic powder may be added thereto.
The overcoat layer is formed by applying the powder material paste onto an electrode and the like, followed by firing. In this case, a firing temperature is limited to 600° C. or less in order to prevent a situation in which the characteristics of the electrode and the like deteriorate owing to a reaction between the electrode and the like and a powder material. Therefore, the powder material (powder material paste) is required to be able to be fired at a temperature of 600° C. or less. In addition, the powder material is also required not to cause warpage of a substrate and not to be easily peeled off from the substrate after the firing.
As a powder material satisfying the above-mentioned required characteristics, PbO—B2O3—SiO2-based glass has hitherto been used (see Patent Literature 1).
In recent years, a reduction in a substance of concern, for example, a reduction in PbO has been promoted from the viewpoint of environmental protection, and various lead-free glasses have been proposed instead of the PbO—B2O3—SiO2-based glass. For example, in each of Patent Literatures 2 to 4, there is a disclosure of Bi2O3—B2O3—ZnO-based glass.